The present invention relates to an oxide superconducting device obtained by combining an oxide superconductor with a semiconductor.
Oxide super conductors formed on SrTiO.sub.3 (strontium titanate) substrates have been discussed in Jpn. J. Appl. Phys. 26 (1987) pp. L1248-L1250.
An oxide superconducting device having a junction structure composed of an oxide superconductor with a semiconductor different therefrom only in oxygen content has been disclosed in Jap. Pat. Appln. Kokai (Laid-Open) No. 63-239990.
On the other hand, SrTiO.sub.3 doped with Nb has been discussed in Phys. Rev. 148 (1966) pp 280-286.
Oxide superconductors are selective in substrates, and sufficient superconducting properties can be attained only on a specific insulator substrate, e.g., a substrate of above SrTiO.sub.3, MgO or YSZ (an abbreviation of yttrium stabilized zirconia,), or an Al.sub.2 O.sub.3 substrate.
When a SrTiO.sub.3 substrate is used, there can be attained junction of an oxide superconductor having excellent joining characteristics, with SrTiO.sub.3. But SrTiO.sub.3 is an insulator and no carrier exists therein. Therefore, although SrTiO.sub.3 can be used as a substrate, i.e., support, for composing a device, SrTiO.sub.3 cannot be positively utilized as a constituent having a function as element.
Accordingly, as described above, for example, semiconductors obtained from oxide superconductors by adjusting the oxygen content have heretofore been used for realizing an oxide superconducting device comprising an oxygen superconductor and a semiconductor combined therewith. An oxide superconductor undergoes a phase transition from superconductor phase to semiconductor phase when its composition is controlled. For example, Y--Ba--Cu--O, a so-called Y series superconductor, undergoes a phase transition from superconductor phase to semiconductor phase when the oxygen content is adjusted. Therefore, the above-mentioned oxide superconducting device comprising an oxide superconductor and a semiconductor combined therewith has been produced by properly controlling the oxygen content by heat treatment or plasma oxidation.
However, our investigation revealed the following facts. The above configuration involves a problem in that with a change of the oxygen content with the lapse of time, the position of a junction of the superconductor with the semiconductor changes with the lapse of time, or it involves a step difficult to control, i.e., heat treatment and therefore, for example, when a superconductor region and a semiconductor region are formed on the same plane, several-micrometers-order control of the positions of these regions is difficult. Furthermore, according to our investigation, the carrier concentration of the semiconductor phase of the oxide superconductor is about 1.times.10.sup.21 (1/cm.sup.3) which is high for a semiconductor, and therefore it is expected that when the semiconductor is joined to the oxide superconductor, the tunnel phenomenon is dominant in the resulting current-voltage characteristics at low temperatures of 100.degree. K. or lower. In addition, there has not yet been obtained a metal/semiconductor junction device which exhibits rectifying characteristics at low temperatures of 100.degree. K. or lower.
According to the configuration described above, when SrTiO.sub.3 is doped with Nb, a mobility .mu. of 600 (cm.sup.2 /V/sec) is attained at a carrier concentration n of 1.7.times.10.sup.20 (1/cm.sup.3) and a temperature of 4.2.degree. K. However, although such characteristics of simple SrTiO.sub.3 doped with Nb have been known, characteristics, in particular, joint boundary surface characteristics, of a product obtained by combining Nb-doped SrTiO.sub.3 with an oxide superconductor have not been known at all.